The subject invention relates to a gun barrel particularly suited for use at elevated temperatures and to the method of making such gun barrels.
It is known that gun barrels become heated during prolonged use. As use at higher and higher temperatures occurs the steels from which gun barrels are conventionally made will suffer substantial reductions in strength and further will be less dimensionally accurate due to greater thermal expansion.
Gun barrels may become heated through a number of mechanisms. One such mechanism is the prolonged use of the gun over extended periods at significant firing rates.
A gun barrel can also become heated in relatively shorter time when used to fire projectiles at more rapid rates.
Gun barrels also become heated where the energy employed in firing a projectile is increased as in developing higher muzzle velocity of a projectile. Where larger amounts of propellant, or where propellant which yields higher energy on burning, are employed this also can lead to more rapid heating of the gun barrel from which the projectiles are delivered.
The rapid heating of a barrel during rapid fire of projectiles for extended times can occur although the propellant energy is relatively low. This heating is due in part to the heat of friction generated as the numerous projectiles are accelerated along the barrel and in a rapid fire sequence.
The mode of failure of structures designed for specific end uses such as gun barrels can be determined by basic mechanisms. One such mechanism is the rate at which heat can be transferred from the internal structure, which receives the heat, through the wall structure and to an outer surface which can dispel the heat. For example, in a gun barrel the heat is received by the barrel at the barrel interior due to the burning and heat of burning of the propellant material. In addition, frictional force of the projectile moving along and against the surface of the interior of the barrel can generate heat at the immediate surface contacted by the projectile. Where the amount of heat which can be removed from the barrel through normal conduction mechanism is limited this places a limit also on the application which can be made of the gun. If barrel temperatures become excessive, the gun may fail. This may occur either locally at the inner surface of the gun barrel by localized melting or metal deformation at high temperature, or throughout the barrel as the physical properties of the overall structure of the barrel deteriorates. Such deterioration can result in a rupture of the barrel.
Another mode of failure of a barrel can be mechanical in nature. Such mode can result from a simple mechanical failure to contain the mechanical forces which are applied on the gun barrel. For example, as a propellant is ignited and burns it generates not only heat but also very high pressure and this pressure must be mechanically contained by the barrel. Also, where the projectile leaves its cartridge and starts down the barrel the rifling on the barrel mechanically applies a torsional force to the projectile to give it spin necessary to aid it in its accurate flight to a destination or target. Where the mechanical force needed to initiate rotation of the projectile is excessive, mechanical failure of the barrel can occur at the location adjacent to the chamber where the barrel rifling starts.
Regarding the heat generated at the bore of a gun barrel this heat can build up very rapidly in spite of the fact that the heat can be transferred through the wall of the barrel to the barrel exterior because of the higher rate at which heat can be produced at the bore compared to the rate at which the produced heat can be carried by heat conduction through the thickness of the barrel wall. For a barrel wall of lower conductivity, when long bursts of firing occur, or when the heat produced by the gases is relatively high, this heat production is concentrated at the bore surface and may not be conducted from the bore rapidly enough because of the limitations in the conductivity of heat through the material of the barrel wall.
There is a heat sink effect in the thickness of the barrel but this heat sink is available only until the temperature of the barrel itself is raised by production of heat within the bore which is in excess of the quantity of heat which can be conducted through the wall thickness based on the characteristics of the material of the wall itself.
In fact the combined barrel and propellant must be treated as a system because all the elements of the gun must be kept in balance. Any one element which is out of balance with the others can cause failure. For example, if the propellant generates excessive pressure or temperature or is used in excessive amount, this alone could disrupt the balance between the several components of the system and lead to excessive heat and thermal degradation of the barrel or bore surface.
It is recognized in the industry that if guns are designed to fire projectiles at significantly higher velocity and at higher energy, better gun barrels will be needed.
To accommodate such higher temperatures generated in the barrels of guns it is theoretically possible to form the barrels of metals which withstand higher temperatures than the low alloy carbon steels conventionally used in forming gun barrels.
Commercially available metal alloys for high temperature applications providing high strength consist of nickel or cobalt-base alloys or refractory metals and their alloys. The commercially available nickel and cobalt alloys generally have greater thermal expansion coefficients than do steels. For this reason they are largely unsuitable for barrel jacket materials if a material having a combination of low thermal expansion and high strength at high temperatures is sought as is the case in the subject invention.
Some iron-nickel alloys such as Invar or IN-907, are alloyed to take advantage of certain magnetic interactions that lead to low expansion behavior over a range of temperatures generally below 500.degree. C. However, these alloys are typically weak compared to steels and have quite low elastic moduli in the temperature range of low expansivity.
The refractory metals and particularly tungsten, tantalum and molybdenum meet the criteria of low expansivity at high temperature, and of retaining high strength but their cost and high density ranging from 10 to 19 grams per cubic centimeter are excessive for practical considerations.
Presently prior art gun barrels are made of low alloy carbon steels. Some gun barrels are made with chromium liners which are electroplated on the inside diameter of the low alloy carbon steels normally used as the jacketing material of current gun barrels. Such chromium lined barrels are adequate for a lower range of projectile firing rates and a lower level of propellant energies. However, as the firing rates and propellant energies are increased as discussed above the basic low alloy carbon steel barrels will prove progressively inadequate.
It is projected that if suitable barrels were available, up to three times as much propellant energy will be employed as compared to current usage to achieve the designed projectile firing rates and muzzle velocities. Since present low alloy carbon steel barrels, and even such barrels lined with chromium, may show failure due to localized melting, the increased energy released in the barrels and applied to the bore of the barrels makes the chromium lined low alloy carbon steels inadequate for the higher firing rates and higher propellant energies to be used.
As indicated above the higher operating temperatures contemplated for proposed gun designs will put severe demands on the gun system. Whole barrel temperatures may be expected to be in the 750.degree. to 950.degree. C. temperature range. High strength of the jacket material at high temperature is essential. Greater thermal fatigue resistance is needed due to increased thermal excursions that the guns will experience. To maintain barrel stability the elastic modulus of the jacket at elevated temperatures will need to be greater than that of the present low carbon steels.
The expansion of barrel dimensions due to thermal excursions in present and prior art alloys prevents long burst firing because as the bore of the gun expands the projectile does not interact desirably with the rifling in the bore and does not attain the proper spin or trajectory. Trajectile accuracy for barrels operating with higher projectile firing rates and at higher propellant energies will be an important criterion nevertheless.
One gun barrel for use at higher operating temperatures and a process for its manufacture is disclosed in U.S. Pat. No. 4,409,881 issued Oct. 18, 1983.